Enterovirus 71 3C Promotes Apoptosis through Cleavage of PinX1, a Telomere Binding Protein.

Enterovirus 71 (EV71) is an emerging pathogen causing hand, foot, and mouth disease (HFMD) and fatal neurological diseases in infants and young children due to their underdeveloped immunocompetence. EV71 infection can induce cellular apoptosis through a variety of pathways, which promotes EV71 release. The viral protease 3C plays an important role in EV71-induced apoptosis. However, the molecular mechanism responsible for 3C-triggered apoptosis remains elusive. Here, we found that EV71 3C directly interacted with PinX1, a telomere binding protein. Furthermore, 3C cleaved PinX1 at the site of Q50-G51 pair through its protease activity. Overexpression of PinX1 reduced the level of EV71-induced apoptosis and EV71 release, whereas depletion of PinX1 by small interfering RNA promoted apoptosis induced by etoposide and increased EV71 release. Taken together, our study uncovered a mechanism that EV71 utilizes to promote host cell apoptosis through cleavage of cellular protein PinX1 by 3C. IMPORTANCE: EV71 3C plays an important role in processing viral proteins and interacting with host cells. In this study, we showed that 3C promoted apoptosis through cleaving PinX1, a telomere binding protein, and that this cleavage facilitated EV71 release. Our study demonstrated that PinX1 plays an important role in EV71 release and revealed a novel mechanism that EV71 utilizes to induce apoptosis. This finding is important in understanding EV71-host cell interactions and has potential impact on understanding other enterovirus-host cell interactions.

A sorting signal suppresses IFITM1 restriction of viral entry.

The interferon-induced transmembrane proteins (IFITMs) broadly inhibit virus infections, particularly at the viral entry level. However, despite this shared ability to inhibit fusion, IFITMs differ in the potency and breadth of viruses restricted, an anomaly that is not fully understood. Here, we show that differences in the range of viruses restricted by IFITM1 are regulated by a C-terminal non-canonical dibasic sorting signal KRXX that suppresses restriction of some viruses by governing its intracellular distribution. Replacing the two basic residues with alanine (KR/AA) increased restriction of jaagsiekte sheep retrovirus and 10A1 amphotropic murine leukemia virus. Deconvolution microscopy revealed an altered subcellular distribution for KR/AA, with fewer molecules in LAMP1-positive lysosomes balanced by increased levels in CD63-positive multivesicular bodies, where jaagsiekte sheep retrovirus pseudovirions are colocalized. IFITM1 binds to cellular adaptor protein complex 3 (AP-3), an association that is lost when the dibasic motif is altered. Although knockdown of AP-3 itself decreases some virus entry, expression of parental IFITM1, but not its KR/AA mutant, potentiates inhibition of viral infections in AP-3 knockdown cells. By using the substituted cysteine accessibility method, we provide evidence that IFITM1 adopts more than one membrane topology co-existing in cellular membranes. Because the C-terminal dibasic sorting signal is unique to human IFITM1, our results provide novel insight into understanding the species- and virus-specific antiviral effect of IFITMs.

Identification of an endocytic signal essential for the antiviral action of IFITM3.

Members of the interferon-induced transmembrane (IFITM) protein family inhibit the entry of a wide range of viruses. Viruses often exploit the endocytosis pathways to invade host cells and escape from the endocytic vesicles often in response to low pH. Localization to these endocytic vesicles is essential for IFITM3 to interfere with the cytosolic entry of pH-dependent viruses. However, the nature of the sorting signal that targets IFITM3 to these vesicles is poorly defined. In this study, we report that IFITM3 possesses a YxxΦ sorting motif, i.e. 20-YEML-23, that enables IFITM3 to undergo endocytosis through binding to the µ2 subunit of the AP-2 complex. IFITM3 accumulates at the plasma membrane as a result of either mutating 20-YEML-23, depleting the µ2 subunit or overexpressing µ2 mutants. Importantly, blocking endocytosis of IFITM3 abrogates its ability to inhibit pH-dependent viruses. We have therefore identified a critical sorting signal, namely 20-YEML-23, that controls both the endocytic trafficking and the antiviral action of IFITM3. This finding also reveals that as an endocytic protein, IFITM3 first arrives at the plasma membrane before it is endocytosed and further traffics to the late endosomes where it acts to impede virus entry.

TRAF3IP3 Is Cleaved by EV71 3C Protease and Exhibits Antiviral Activity.

Enterovirus 71 (EV71) is one of the major pathogens of hand, foot, and mouth disease, which poses a major risk to public health and infant safety. 3C protease (3Cpro), a non-structural protein of EV71, promotes viral protein maturation by cleaving polyprotein precursors and facilitates viral immune escape by cleaving host proteins. In this study, we screened for human proteins that could interact with EV71 3Cpro using a yeast two-hybrid assay. Immune-associated protein TRAF3 Interacting Protein 3 (TRAF3IP3) was selected for further study. The results of co-immunoprecipitation and immunofluorescence demonstrated the interaction between TRAF3IP3 and EV71 3Cpro. A cleavage band was detected, indicating that both transfected 3Cpro and EV71 infection could cleave TRAF3IP3. 87Q-88G was identified as the only 3Cpro cleavage site in TRAF3IP3. In Jurkat and rhabdomyosarcoma (RD) cells, TRAF3IP3 inhibited EV71 replication, and 3Cpro cleavage partially resisted TRAF3IP3-induced inhibition. Additionally, the nuclear localization signal (NLS) and nuclear export signal (NES) of TRAF3IP3 were identified. The NES contributed to TRAF3IP3 alteration of 3Cpro localization and inhibition of EV71 replication. Together, these results indicate that TRAF3IP3 inhibits EV71 replication and 3Cpro resists such inhibition via proteolytic cleavage, providing a new example of virus-host interaction.

[Identification of the Transcriptional Activity Domain of EV71 3D(pol)].

Enterovirus 71（EV71）is one of the major pathogens of hand, foot and mouth disease (HFMD). The EV71 genome encodes an RNA-dependent RNA polymerase（RdRp）,3D(pol),which is critical for genome transcription and translation. However, how the 3D(pol) interacts with the host remains unclear. Yeast two-hybrid systems provide an effective approach for detecting protein-protein interactions. In this report, we inserted the DNA sequence of 3D(pol) into the pGBKT7 vector as the bait plasmid for the yeast two-hybrid experiment and transformed the plasmid into the yeast AH109 strain. We detected the expression,cytotoxicity and self-activity of 3D(pol).The 3D(pol) expressed well without affecting cell growth but exhibited strong transcriptional activation in yeast cells. We further constructed a series of pGBKT7-3D(pol) deletion mutants and identified the shortest transcriptional activation domain（1-94aa）using a self-activation assay. The results provide a molecular basis for screening the host proteins that interact with 3D(pol) using the yeast two hybrid system.